The longest time you'll ever have to wait.
(e.x.) I'll be out in just a second!
*comes out six years later*
20π 1π
An indeterminate amount of time between now and never where time is both not and of the essence.
Hana: Can I have your help with shoveling my car out of the snow, James?
James: Just a second and I will be right there I'm a little busy at the moment.
Hana: On second thought I'll just do it myself, thanks anyway.
A length of time, generally used by someone who is going to take much longer than one second.
Come on mom, you said just a second three hours ago.